Inhalation and/or ingestion of amosite asbestos fiber in occupational settings is associated with increased neoplastic risk. Recent data suggest that indirect occupational and household exposures may also increase the risk of developing asbestos-related diseases. Amphibole minerals occur with both asbestiform and non-asbestiform habit. Amosite (asbestiform grunerite) has been mined as asbestos, and grunerite is present in a number of common rock types, which may upon pulverization release fibers into the air. While amosite is known to produce excess malignant disease, the biological potential of non-asbestiform grunerite is unknown. The aims of this research are to examine in detail specimens of this mineral, in its range of habits, from the extensive collections of the American Museum of Natural History and from other sources. A thorough understanding of this mineral group and the interrelationships with form and occurrence is desired. Specifically, the intent is (1) to determine the physical-chemical properties of both fibrous and non-fibrous grunerite, with special attention to the mechanisms responsible for the formation of fibers, (2) to formulate a hypothesis suggesting which properties impart biological activity and which amphibole habits manifest them, (3) to establish reference minerals for biological experimentation. To achieve these goals, the specimens will be studied from the macroscopic to the most minute scale using a wide range of techniques: optical microscopy of thin sections of grunerite-bearing rocks and mineral grains; X-ray diffraction of powders (using Gandolfi and Guinier cameras and powder diffractometry); chemical analysis of these materials using an electron microprobe; SEM analysis fo fragments and fibers; analytical HRTEM analysis of thinned crystals and fibers. The results will be a far better understanding of grunerite in its asbestiform (amosite) and non-asbestiform varieties, necessary for modeling factors of biological activity. The highly characterized specimens will be invaluable for biological studies.